Field of the Invention
The present invention relates to a process for producing a honeycomb structure-extruding die having excellent wear resistance by coating a surface of a base material made of stainless steel through a CVD technique. The base material is adapted to be used for the die and the CVD technique is preferably carried out at a medium temperature
It is known from, for example, Japanese patent application Laid-open No. 60-145,805 (extruding die and production method thereof, Corning glass works) that wear resistance of a honeycomb structure-extruding die is improved by treating a surface of a base material for the die using a CVD process.
It is also disclosed, for example, in Japanese patent application Laid-open No. 61-69,968 (steel matrix, producing process thereof, Berna AG Olten) to utilize the CVD process for a homeycomb structure-extruding die base material made of stainless steel.
However, when the die base material is to be coated by the CVD process, it is indispensable to remove contaminants, a processed layer, an oxidized film, etc. from the base material to be coated, by fully washing the base material before the coating. If this step is not satisfactorily effected, a coating film is not formed or the coating formed is peeled during cooling. Thus, even if the coating film is formed, such a film has only a short life. Particularly, if the base material for the honeycomb structure-extruding die is stainless steel, it is likely that an oxidized film is formed on the surface and particular care needs to be made to peel the oxidized film. An acidic solution is usually used to remove such an oxidized film. However, this process suffers a problem that since the resulting die is rinsed with water to remove the acidic solution remaining on the surface after being washed with the acidic solution, and the die is dried in an oven or the like, an oxidized film is formed again during the drying, and a good coating film cannot be obtained by the CVD process.
The above peeling problem can be solved by ensuring a relatively long cooling time after the coating by the CVD process. However, if the cooling time is made longer, the total coating time by the CVD process increases, resulting an increased cost in producing the die. On the other hand, if a material, which is required to be hardened, is coated by the CVD, the coated material needs be quenched after the coating, so that it is substantially impossible to prolong the cooling time.